1. Field of the Invention
The invention relates to power bandpass filters produced by electromagnetic resonance cavities, and more particularly to the coupling structures used to produce high-performance bandpass filters with an elliptical frequency response.
2. Description of the Prior Art
Cavity bandpass filters are used in terrestrial television transmission systems, and more particularly in transmitters operating with frequencies between 40 MHz and 1 GHz. In this frequency range, and for a power between several watts and several tens of kilowatts, these cavities are of the coaxial type.
A television transmission system uses a certain number of bandpass filters, each filter having a passband corresponding to a transmission channel. It therefore allows a narrow band of frequencies to pass, corresponding to a channel without attenuation while blocking the frequencies outside this band.
Cavity bandpass filters are constructed by coupling a certain number of cavities together. The desired order of the filter is obtained by associating several cavities in series. Thus, a second-order Chebyshev bandpass filter is obtained with a single cavity, a fourth-order filter is obtained with 2 cavities, and generally a filter of order 2N is obtained with N cavities.
A coaxial cavity is composed, for example, of an outer conductor of square section and a cylindrical inner conductor. These two conductors are connected at one end by a short-circuit plate, the other end of the inner conductor of length L is free, therefore in an open circuit. If it is excited by an electromagnetic field, this system behaves like an RLC circuit tuned to the frequency F0, where F0 depends on the length L of the conductor:L≈pλ0/4 with: p=1, 3, . . . 2n+1 and λ0=c/F0 
Thus the in-series association of these cavities can be obtained by producing a coupling between the cavities in various ways, such as, for example, an aperture in the wall common to the 2 cavities or by means of a conventional coupling loop.
FIG. 1 shows a basic bandpass filter of order 8 obtained with 4 cavities. The filter is composed of cavities 1 to 4 juxtaposed and coupled together by means of conventional coupling loops C12, C23 and C34, connecting the cavities 1 to 2, 2 to 3 and 3 to 4 respectively in series. An input signal Sin enters the first cavity through an input coupling element, then propagates into the second cavity, the third cavity, and the fourth and last cavity. A filtered signal Sout leaves this last cavity through an output coupling element.
To obtain a conventional Chebyshev filter, the N cavities are simply associated in series and the type of coupling used to couple the cavities to each other is of no importance. The curve obtained with this type of filter is shown in FIG. 2. This transmission curve (1) shows an example of a bandpass function in which the attenuation is very low (point M21) at the central frequency F0 of 2000 MHz, while only at the frequencies of 190 MHz and 210 MHz is the attenuation close to −30 dB (points M22 and M23).
Yet communications systems demand high-performance filters for which the attenuation is low in the passband and this attenuation is very high outside the passband. The transition areas between the areas of low attenuation and high attenuation must be as narrow as possible.
The larger the number of cavities, the steeper the sides of the response curve in the transition areas and the higher the performance of the filter. But the addition of cavities increases the insertion loss, the size, the weight of the filter and the complexity of adjustment.
A microwave filter is described by document EP 0 878 862. This elliptical-response filter comprises complementary coupling means to produce insertion zeros at determined frequencies in the frequency response curve. These insertion zeros are created by the complementary coupling elements constituted by the probes 120, 124.
The invention therefore proposes a topology for a high-performance coaxial cavity bandpass filter with an elliptical response comprising transmission zeros so as to limit the transition areas.